1. Field of the Invention
The present invention relates to a polypropylene resin composition. More specifically, it relates to a polypropylene resin composition composed mainly of a crystalline ethylene-propylene copolymer and capable of providing a molded article having a particularly good balance between the thermal deformation resistance and the impact resistance thereof and having superior mechanical characteristics and coating properties, and further, having a superior moldability. This polypropylene resin composition can be utilized in the fields of, for example, large-sized components for automobiles and components for domestic appliances.
2. Description of the Related Art
A crystalline polypropylene is now utilized for various molded articles which must have physical properties such as a strong rigidity and a high thermal deformation resistance, but this material has unsatisfactory low temperature characteristics and a poor impact resistance and coating property, because it is a high crystalline polymer; this can cause a problem in that it cannot be utilized for applications where a strong rigidity, high thermal deformation resistance and impact resistance, and a good coating property are simultaneously required. To improve the impact resistance and coating resistance of the crystalline polypropylene, the use of a polypropylene comprising a crystalline ethylene-propylene block copolymer as the crystalline polypropylene and having an amorphous ethylene-propylene copolymer blended therewith, and a polypropylene composition comprising a crystalline ethylene-propylene block copolymer and having an amorphous ethylene-propylene copolymer and various kinds of polyethylene blended therewith has been proposed.
These proposed conventional polypropylene compositions have the disadvantage of causing a lowering of the above-described properties, such as the rigidity and thermal deformation resistance, inherent to the crystalline ethylene-propylene block copolymer.
To alleviate the disadvantage of the proposed conventional polypropylene compositions, Japanese Unexamined Patent Publication (Kokai) No. 53-64257 describes an invention entitled "Impact-Resistant Resin Composition." This publication discloses a resin composition having an improved low temperature impact resistance, which composition comprises a crystalline ethylene-propylene block copolymer, an amorphous ethylene-.alpha.-olefin copolymer, and talc. Further, this publication provides descriptions and working examples to the effect that this composition shows a marked improvement in the low temperature impact strength, rigidity and coating property thereof, compared to the conventional crystalline crystalline-propylene block copolymer. Even in the above-described composition, however, a satisfactory balance between the rigidity and the impact strength is not obtained, and the coating property is also unsatisfactory. Accordingly, a molded article prepared from the above-described composition can be used for applications in which the appearance is not important, but is unsuitable for use in applications where not only the various mechanical properties but also the appearance is regarded as important, such as when used in various parts for automobiles, particularly instrument panels and bumpers.
Compositions comprising an olefin resin and a hydrogenated block copolymer, each used in the present invention, have been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 58-206644, 58-215446, 59-223745, 60-166339, 61-28547, 61-28548, 61-34047, 61-34048, but these compositions contain large amounts of a hydrogenated block copolymer and softening agent, and thus the rigidity thereof is low. Therefore, the technical concept of the above-described proposals is entirely different from that of the present invention, i.e., a technical concept such that a hydrogenated block copolymer and an inorganic filler are incorporated in a crystalline ethylene-propylene block copolymer to thereby provide a molded article having a better balance between the thermal deformation resistance and the impact resistance, and a superior coating property and appearance.
Further, Japanese Unexamined Patent Publication (Kokai) No. 57-51735 proposes a composition comprising a crystalline ethylene-propylene block copolymer, a hydrogenated block copolymer and an amorphous ethylene-propylene copolymer, as used in the present invention, but this composition does not contain an inorganic filler, and accordingly, the thermal deformation resistance and rigidity thereof are low, and thus the object of the present invention cannot be attained.
Therefore, the molding of the conventional polypropylene resin compositions does not provide a molded article having a good balance between the thermal deformation resistance and the impact strength, a superior mechanical property, and a superior coating property.